Rheumatoid arthritis (RA) is a chronic systemic inflammatory disease with peripheral synovitis as its main manifestation. The presentation of the disease and the progression is highly variable both within and between the individuals. The disease is thought to occur as an immunological response to an as yet unidentified antigen. The prevalence of RA is 1 to 2 percent of the general population and is found worldwide. Females with RA outnumber males by a 3:1 margin. The symptoms and the signs of the disease may vary from joint complaints like pain, stiffness and functional impairment, to more constitutional complaints like fatigue and detrimental to general health. The current paradigm for rheumatoid arthritis suggests that persistent synovitis leads to erosive joint damage, progression of which results in functional disability. Because of this variety in disease expression a huge number of outcome variables have been used in the past decades to evaluate interventions in clinical trials.
Diagnostic and prognostic markers are relevant tools in the management of many medical conditions because they help to identify and stratify patients into different risk groups, enabling treatment to be targeted appropriately in order to reduce morbidity and mortality. An evidence-based approach to identify the most valuable diagnostic markers for a given disease is clearly important. Biological markers will play an important role in the development and the early monitoring of disease modifying anti-rheumatic drugs with respect to future radiographic progression.
The diagnosis of rheumatoid arthritis has been hampered by the lack of a truly disease-specific serologic marker and thus diagnosis of RA is mainly based on clinical criteria recommended by the American College of Rheumatology. Studies have shown that 40-83% of subsequent progression of rheumatoid arthritis can be predicted by a combination of prognostic factors such as joint involvement, high levels of C-reactive protein and RF positivity. There are similar findings for predictors of functional disability in studies. Till date the most consistent diagnostic feature has been RF positivity, which is equally important in predicting joint damage and functional disability. Immunoglobulin A RF and the co-presence of RF with anti-keratin or anti-filaggrin antibodies may increase levels of prediction. Other potentially useful antibodies include anti-RA33 autoantibodies and antibodies to the stress protein BiP, which seem to have higher specificity than RF for predicting RA outcome. Apart from being promising diagnostic markers these autoantibodies or the underlying cellular autoimmune reactions, respectively, may also play a role in the pathogenesis of RA (Steiner G, Smolen J S. Z Rheumatol. 2002 December; 61 (6): 667-73). Added value of genetic predictors over that of RF remains inconclusive.
To date, only the rheumatoid factor and anti-filaggrin antibodies (including anti-stratum corneum or “anti-keratin” and anti-perinuclear factors) have been used with sufficiently acceptable standards for diagnostic purpose. IgM rheumatoid factors can be detected in about 80% of patients with rheumatoid arthritis but they lack specificity since they are also found in other autoimmune conditions (lupus, Sjogren's syndrome), in chronic infections, and in certain lymphoproliferative syndromes (with or without cryoglobulinemia). Anti-filaggrin antibodies are more specific (70 to 100% depending on the study) but can only be detected in 30 to 50% of the patients. High titers of rheumatoid factor (IgM and/or IgA) and anti-filaggrin antibodies are factors of poor prognosis because they are associated with destructive polyarthritis, sometimes complicated with extra-articular signs (nodules, vasculitis). Antigens derived from filaggrin have been used for their diagnostic purposes in rheumatoid polyarthritis (U.S. Pat. No. 6,890,720). Among the new autoantibodies being studied, only anti-Sa appears to have real diagnostic and prognostic value but the recent data must be confirmed (Sibilia J, Presse Med. 2000 Oct. 21; 29 (31): 1723-30).
Moreover, first described as a marker for RA in 1964, anti-perinuclear factor (APF) was directed to constituents of the keratohyaline granules later found to contain the protein filaggrin. Despite its specificity for RA, because of exacting technical requirements, APF never became widely used. Anti-keratin antibodies (AKA), first described in 1979, bound filaggrin bound keratin in senescent esophageal cells. As was APF, AKA had greater specificity for RA than RF. Anti-perinuclear factor (APF) and anti keratin antibodies (AKA), two tests known for a long time, have a high specificity of up to 70% for RA. The tests are done by immunofluorescence but did not become popular in clinical practice, despite high specificity, due to various technical difficulties in performing the assays. Filaggrin was identified as the antigen that was targeted by both these autoantibodies. Antibodies to Sa antigen have also been detected in sera of patients with RA but its association with RA has not been confirmed. Besides Peptides immunoreactive with autoantibodies from patients with rheumatoid arthritis (U.S. Pat. No. 6,858,438), autoantibodies from a body fluid that react with a microtubule organizing center (MTOC) (U.S. Pat. No. 6,638,723), measurement of depressed activity of catalytic antibodies (U.S. Pat. No. 6,130,049) have also been suggested as diagnostics of rheumatoid arthritis.
Mannose binding lectin (MBL) is an acute phase serum protein that has a significant role in innate immunity. This C type lectin with specific binding affinity to mannose and N-acetyl glucosamine (GlcNAc) is structurally homologous to C1q, the component to classical complement pathway (Holmskov et al, Immunol Today 1994; 15: 67-74). On binding to the specific carbohydrate, its associated serine proteases (MBL associated serine proteases or MASP) get activated leading to activation of complement cascade; popularly described as lectin pathway. Consequently it has significant role in eliciting the inflammatory response and thus it has been well associated with the pathogenesis of RA. Particularly in RA, agalactosylated IgG (IgG0) that has an exposed GlcNAc can be an easy target for MBL binding leading to generation of inflammatory response (Malhotra et al, Nat Med 1995; 1: 237-243).
Recently in Systemic lupus erythematosus (SLE), a related autoimmune disorder, autoantibodies against MBL have been reported (Seelen M A et al, Clin Exp Immunol 2003; 134:335-343, Takahashi R et al Clin Exp Immunol 2004; 136: 585-590). These anti-MBL autoantibodies in the sera of SLE patients have been shown to decrease the functional activity of MBL. The presence of anti-MBL autoantibodies in the sera of RA patients was determined for the first time in the present invention and thus represents a genuine discovery of a diagnostic and prognostic marker for RA. Our research makes it possible to demonstrate for the first time the presence of autoantibodies directed against MBL in serum of subjects suffering from RA.